I’m thinking: a Shimano HG61 cassette (for 29ers), giving you a 36-tooth big ring at the back. Coupled with an E Thirteen XCX chain guide. I don’t know much about chain guides, so is this the best out there for a low budget?

Also, can you keep the chainline the same without moving the front middle ring on the crank arm?

I’ve ordered the Superstarcomponents one. At £17 its got to be worth a go. The paulscomponents one looks spot on, but is a bit pricey for what it is. Chainline ? Wont be any different to running 3×9 I would have thought.

If you are looking for cheap you can just remove the cable and adjust the limit screws on your front mech and use that.

On the chainline, do you want to use the big ring up front as the single ring? Or the middle ring?

If the middle ring then the chainline will be spot on already, if using the big ring then you can just shift it over to the middle ring position. You will need shortened chainring bolts though, or just take a file to the ones you already have.

the superstar guide is half the price of the e13 one and virtualy identical mine is very good , light and will fit up to a 42 tooth ring

cheapest is take the inner and outer ring off the chainset, adjust limit screws on your front mech so it won’t shift out of the middle ring, job done

even easier just don’t shift from the middle ring 0 mechanicing, 0 cash outlay

invoice for this advice will be in the post

on a serious note, get an unramped front ring (renthal has good long teeth but will work with a standard chain) shorten the chain as you won’t need the length, lose the front mech and shifter and that’s it, got this set up on two bikes, no chain drop in the last 12 months on local trails and riding in Scotland. you don’t need to replace the weight you’ve taken off the bike by adding an unnecessary chain device