The Heiland TRD2 is pretty small. X Rite used to make (not sure if they still do) portable densitometers both for reflection and transmission that were pretty small, you were able to put them on your pocket.
If you do an X Rite search on e bay on the home page you will get results for the best ones available. The ones you are looking for are, X Rite 361T, 369T, 310T (these are only transmission B&W) or the 810 or 811 which are both transmission and reflection color densitometers. If you plan to do alt printing, then the ones you need are the 361 or 369 as they both have an UV channel. If all you plan to do is silver then the color densitometer with both transmission and reflection capablities are the best.