I've built my own looms before, usually by collecting up bits of other looms and grafting them together. Here's a few tips:
Its very important to use a recognised colour code standard, (preferably the Mini one) because it makes it so much easier to trace faults.

Do NOT even consider using those red , blue and yellow crimp on terminals, they are so low quality that it should be illegal to sell them. Preferably solder or use high quality crimp terminals with the correct crimp tool.

Heatshrink is very useful to repair insulation if you have to join wires, get the dual wall type that is lined in hotmelt glue, its much better than the thin stuff.

Don't use electrical tape to wrap the loom, even the highest quality electrical tape won't stand up to heat and fumes under the bonnet and will fall off or go gummy very quickly. If you aren't fussed about originality I like spiral wrap, it works well and is easy to undo, otherwise proper loom wrapping friction tape (it doen't have any glue on it) is the best thing to use. On an early 850 the loom should be braided, you can have braiding applied to your own loom by most of the loom making specialists.

Or just buy one from one of the specialists, most will tailor one to suit any changes you want to make.

Tim