I'm assuming you can't use hiragana since you said you want to record romaji for "technical purposes", but if you can help it, I'd really recommend going with hiragana. You can download a set of reclists (one of which is 8 mora)
here.
However, if you really want to record romaji though, you could try the
VCV generator, change "文字数" to 8 rather than 6, and then change the hiragana characters under "基音素" to romaji, with an
underscore before each sound. If you have tady159's VCV oto generator (I cant seem to find a working link for it), you should be able to generate an oto for the generated list with no problem.
A set of 8 mora guideBGM that you can use can be found
here.