Just make yourself a simple jig, a couple of bits of wood fixed at 90° with a groove in which the drill can sit. Hold it against the wall and you will be ok in both planes. I also have one made at 3° for drilling boiler flue holes to get the right fall.
I had looked at the UJK guide, which looked nice, but was out of production at the time (and expensive) and the Rutland offering that looked a bit flimsy, so I settled for one of these old things that I found on eBay in the US - it was about £15 plus about the same for postage at the time. I had to buy a chuck and found a threaded spigot to connect my own drill.
I find it very useful for sundry perpendicular drilling (the original requirement was drilling into the sides of kitchen cabinets and through into fixed shelves for Confirmat screws).
Or put a ring around the drill bit. Needs to be larger diameter than the drill bit, and up on the shank near the chuck. Drill slowly, and if you are level, the ring will stay in place. If you are not, it will move down or up the bit.