I'd probably start with something like this - an IP67 rated 1x4 keypad. You can add on a single one in a different orientation for the thumb - or use five singles to enable comfortable siting for each individual finger. Embed them in something like polymorph to build an ergonomic mount that can be cable tied to the bars.
... conceivably, you can live with four keys. There's a lot or redundancy in English, and you could possibly do without a lot of letters. You could either leave Barakta to sort out what you did mean, or I'm sure people have worked out relatively simple ways to get an approximation of English out of the other end, by reinserting the missing letters automagically. Assuming that you're just planning on using a small microcontroller, then you'd have to use something like that, because an extensive dictionary lookup approach is unlikely to be sensible with a data space limited device.
Alternatively you could use one of the many alternative typing systems that have been invented over the years for the purposes of faster typing, using one or two hands, and generally a very limited set of keys. This does seem to have been regularly reinvented, so there must be plenty of examples around.
