You HAVE to tell us how you built this wall. Dry stack REAL rock is to die for. Do NOT cheapen what you have done with some...lame cap. What have you been looking at for garden wall caps? There are premade concrete wall caps that are 2'X 1'X 2" thick. Is this what you've cost?
That wall is priceless, if you've not done proper drainage and foundation of compacted gravel...I would redo the wall. Set on compacted gravel, figure the surcharge or reduce the surcharge behind that wall, add a perforated 4" pipe on gravel, at the bottom of your wall covered with landscape fabric, backfilled with drainrock then landscape fabric topped with 6" of soil. Try to slope away from the wall into a small swale between wall and slope. That will help to reduce surcharge...the amount of water the soil behind that wall can accumulate to put pressure on your wall. You could always do the drainage and foundation later. The cap is secured with mastic but won't stop taking the cap off if you need to reconstruct your wall. REAL STACKED STONE? Wonderful.
Wall caps HAVE to be concrete, 2" thick, finely troweled, uniform and be able to accept the top of a wall that is fairly straight and well matched with ledge stones? This is the beauty of a garden wall. This will make your true stone wall worth it's weight in gold. Serious!!
Pricing capstones for CMU walls as well as real stone walls was always amazingly inexpensive compared to the entire wall. To cheap out now would be a shame. I remember a $20 price per capstone at 2'X 1' X 2", uninstalled but not OUR price. What have YOU found? Do NOT even imagine to add color to the cap. Stay with light dove gray. Again trust me on this. I've done hundreds of projects with garden walls and THE ONLY way to go is dove gray! You will regret any color later. I am a Landscape Architect that KNOWS what is value in the landscape and what will be a waste of money. Send a picture, please! 40 feet of cap would be at the most, $400. How much was the stone you used, how high is your wall and is it straight or curved? That is a consideration. Still doable I've got alternatives...