Hi,
Don't do it! Don't fit a square section spring. The gun will hate it. I have tried an 'OX' square spring in mine many years ago. It turned into an absolutely horrible air gun. I reckon a Chinese B2 would shoot more smoothly than an HW35 with a square spring. Honestly, get yourself a standard spring or a V-Mach F1. It will save you having to buy another piston washer and strip it all down again in a couple of weeks time.
Best Regards,
Andy.