We really only pull tires of various sizes. And they do a GREAT job on the front area of the barn entry, leveling, smoothing stuff up.
Not sure what you want a “real drag” for, when tires can do the same job for you and are pretty safe for pulling.
If you are only doing review work in Driving, then you want something that is resistant but not weighty. You don’t want it so heavy she is discouraged in trying to pull it very long or gives up trying to go forward! That is bad.
I would suggest a lighter tire to start, off a medium size car, not a big truck tire and NOT some kind of tractor tire. She won’t have trouble moving this size tire, but it won’t get her sored up fast either because she hasn’t done this pulling for a while.
I would suggest you do review work with her out on the long lines, get the commands straight, get prompt responses. You say Whoa, she STOPS! You want all her voice commands in place, before you EVER think of hooking her to an object behind her. She should do all this work in an open bridle so she can see what is happening, not be frightened while it happens. You can work in an arena, ground drive her on the driveway or around the farm to sight see. Be CAREFUL that YOU are moving fast enough not to be hanging on her mouth, which you can see by walking to one side of her. No reward for being good if mouth is constantly being pulled. Carry a whip all the time, it is your tool to JUST touch her with for going FORWARD, since you have no riding legs. Touch of lash, along with voice command is ALL the whip use she probably needs. Rein slapping on the butt is a TV thing. NOT how you ask for Forward on a Driving horse. Rein slapping on the rump CAN cause kicking.
You probably want to use a singletee to pull the tire and spread the traces. I would suggest you have help on hand to hook the tire for you. Be available for any adjustments or fixing needed. Helper can give horse a lead if needed with tire weight until she is confident, then step away. I do think you should add some heavy string to lengthen the traces and move singletree with tire out behind her, away from her hind feet as she goes along.
I NEVER suggest using fence posts or logs to pull in a training setting. The end tends to catch on rough ground or other things, then may or may NOT pull free. They always JUMP when they get free, which adds an odd feel to the traces. They can really swing outward on circles, but again, the flat end does snag. I consider them to be a poor training tool. Tires are rounded, “give or bounce off” when they hit things, don’t get hung up on things.
You might look up Ground Driving, starting young horses, Long Lining, read the old posts on this Forum, that contain LOTS of information. Not sure how much Driving experience you have, but they cover a lot of ground in the stages of starting a horse. Driving is NOT as easy as it looks, so take your time with your horse, get both of you comfortable with things before moving to the next step. There is NO SCHEDULE of time it takes to get one going. If it takes 90 days, then that is what it takes to get horse RELIABLE for commands and staying calm, before you can hook a tire. You should consider this part as putting on your Basics again, because she is building trust in you and what you ask of her, knowledge of what her part is in responding. She will need this information as she continues her Driving training, to meet and face new situations quietly. Each horse is a bit different in accepting things, so they move at different speeds and none of them are WRONG or make you less skilled as a trainer. My young horse took a VERY LONG time, MONTHS, to get started last year. MUCH repeating of everything, so she could count on it being the same thing, before she could go to the next step. She was quiet and reliable when finally hooked to a vehicle, we had no surprises from her. She then smoothly (one end of summer to the other) moved along in Single training, to work in a Pair by Fall. However there probably won’t need more than a run-thru session on commands after winter off, and I fully expect her to walk right off in the Pair again with no issues. She is SOLID in her training, worth the time investment to gain the work levels we expect from her in the future. This year she will be going out and about, meeting real situations away from home, to learn all the new stuff she will find at shows and competitions.