Setup : cavaletti at X, to be jumped B-E off a 20m circle
Diagonal jump between X and C set so that it can be jumped off the same 20m circle at A that goes over the cavaletti (roughly B-H diagonal but it must be a smooth line off the circle)
Diagonal jump between X and A set so that it can be jumped off the same 20m circle at C that goes over the cavaletti (roughly B-K diagonal, again with the smooth approach off the circle)
I set my diagonals on the centre line about 4-5’ on either side of the cavaletti. Sometimes I will make one of the jumps an oxer. They don’t need to be big jumps; 2’6"-2’9" works quite well.
Exercise:
Step 1: Canter the circle at A on the left rein, going over the cavaletti. The cavaletti should be small enough that the horse can just lift his legs a bit higher without really having to jump - small enough that you do not need to release or close your hip angle. Use this circle to find a good, balanced, jump canter. Allow your horse to find his spot. The right canter allows him to shorten or lengthen stride without having to make a big enough change in pace to upset your balance - you should barely notice it was needed.
Repeat on the right rein on the C circle.
Step 2: Left canter the circle at A over the cavaletti. After two good passes over the cavaletti (ie. you have the good canter and barely notice the cavaletti) on the third circle jump the B-H diagonal. The exercise is to maintain that good canter to the jump and allow him to make any little stride adjustment, without him rushing, and without you dropping him. Easier said than done, especially if your horse likes to jump. 
Repeat on the right rein at C.
Either work one rein several times, or alternate. Depending on how well you and your horse are doing.
Step 3: Ride the circle, cavaletti and diagonal as in step 2 (but one good pass over the cavaletti should be enough by this point), and continue around the end of the ring and jump the other diagonal. This tests your (and your horse’s) ability to maintain that good jumping canter.
If you enjoy such challenges, canter all the way around the ring after the circle and diagonal, jump the same diagonal in the opposite direction and do a bending line to the other diagonal. It is tight, but doable. It is also a long distance to maintain that good canter.
If you feel ambitious, also set up a jump inside each circle along the F-B-M wall. This gives you an outside line and two bending diagonal lines (or straight diagonals if you jump the outside jumps on an angle).
Have fun! 