Sir, I salute you! Doing single sided pcb design without any jumper wires is a black art in itself.
If you can find a dual gang trimmer or 2 simple 25k trimmers, go ahead and use them on the board. The main idea of the original circuit was to get constant loudness while tweaking the balance of your typical hifi speaker amp. Just set both to 50% with a DMM and turn the more silent side up untill both channels match in loudness. Just make sure you use the right tongue angle when tweaking those trimmers.
OH, sweet! two trimmers are easily swapped in, and are eaiser to get... I enjoy the challenge of the single side. I dont know how many times I have had to scrap a PCB design and start over to avoid jumpers.