Here's a well designed buffer by Martin from SoulSonic FX. Use it as a stand alone buffer or as an output buffer in another pedal. Should fit easily in a 1590a enclosure.
This is a library of perfboard and single-sided PCB effect layouts for guitar and bass. I'm not an electrical engineer by any stretch of the imagination, just a DIY'er who likes drawing layouts. It is meant for the hobbyist (so commercial use of any of these layout is not allowed without permission) and as a way to give back to the online DIY community.