I have been using the Tech Shop Filament Guide which works well but I was having problems with the filament un-spooling or jumping off the spool. This guide uses an 8" piece of PFTE tubing (4mm OD/ 2mm ID) to guide the filament which keeps it on the spool. The tubing reaches part way over the top of the spool. This design installs best on printers with an extruded aluminum chassis but may also be used with other printers using the two 5mm mounting holes. Since I installed this guide, I have not had any filament unwind or jump the spool.
I added another filament guide design that uses 1/4" PFTE. This works well if you are using my design for an Airtight Filament Canister below:
The new filament guide allows you to connect the canister directly to the filament guide. I've also found that the larger tubing is good when using a spool. It has a bit less drag.