I love giving ideas for jump gridwork, as well as courses and neat exercises. Here is what I have:
*note I'm not sure what kind of jump building supplies you have (like how many poles etc) so if any of these require more supplies that are available, either improvise or discard the idea*
Double bounce from the trot:
ground pole 9 feet in front of the first vertical, 11-12 feet between first vertical and second vertical (depends on your horse for the distance between the jumps) and than another 11-12 feet between second and third vertical, followed by another ground pole placed 9 feet after the gird.
Approach at a controlled trot, apply aids for canter after the ground pole, canter steady through bounces, land inside the ground pole at the end of the grid and canter away balanced.
This exercise will help with:
Improve the horses sharpness and technique
Improve the horses athleticism
Encourage elasticity in both horse and rider
Ensures rider's independence of balance
Establishes rhythm and consistent pace.
I will post another in a few minutes. Do you do anything with jump fillers live liver pools, flower boxes etc? Have you done square, ascending, descending, triple, and hogs back oxers? Have you done any narrow fences or jumps on an angle? I'm just trying to get an idea of what you have done so I can suggest things a little better. Also are you looking for just your grid work or would exercises involving turns to other jumps be of interest to you?