I’ve owned 5-6 BMSS saddles over the years. Two big points to be made.
Your pad MUST have two pockets on either side of the spine channel to hold inserts.
It must be easily washable in the washing machine that you own NOW.
The BMSS has no internal panels to keep the riders weight off of the horse’s spine. Most newer treeless saddles now have panels built into them to provide some spine clearance. But most of them still require additional lift from an appropriate treeless saddle pad. The BMSS REALLY needs a good treeless saddle pad under it. SKITO pads have the best and widest range of inserts available. No other company will custom make inserts for your pad.
You want a pad that let’s you remove the inserts and leave you with a pad made of material that you can fit into your washer and really clean well. This means that the thick (stiffer) wool felt pads can only go into a large front loader machine and since they have to hang dry they will take quite a while to really dry. Of course you can spend money for 2-3 or those pads perhaps?
Pleeeeze do not base your pad choice on pretty colors or fabric choices. You and your horse may regret it!
Haf pads (Actionridertack.com) have removeable inserts and wash easily. Insert selection is pretty limited but you can always buy better inserts fron Skito and put them into your washable HAF pad. Equipedic, 5-Star and a few others are based on the wool felt. Yes, they may have foam inserts but that felt soaks up and hold the sweat and dirt and becomes nasty (for your horse) quickly.
So look for a pad that is a soft, foldable fabric that fits into a washer easily.
The new Toklat Matrix pads are very washable, some color selections available, different foam inserts available. No customized shims or inserts available from Toklat but Skito shims and inserts can be added to ANY pad that has insert pockets.