KML maps on Google Earth (or other mapping app) on a phone or tablet is certainly one option. In the next release of RM (early Feb) we are moving to vector depth areas (isobaths), which really opens up the options for KML and pre-HDS (non-raster supporting) Lowrance units. Here's an example of a map showing in KML on an Android phone. Note also that performance is (can be) very good, with smooth zoom and fast update. My experience of recent phones is that the GPS performance is surprisingly good - easily good enough for fishing use.
If using a phone or tablet doesn't cut it, the options are more limited. I know that Lowrance do have a range of handhelds (Endura), but have not tried them. Maybe Greg could chime in and let us know about AT5 compatibility there. Perhaps Hubert's suggestion of trying an earlier Atlas format would work?
We would like to support Garmin and we are actively looking at doing so - so in the future, that might be another option as they have a wide range of handheld units.
Matt
Edit to add: Just to be clear, this style of KML map is not yet available within ReefMaster, but it has been created with the current beta version, which will be available very soon. You can still create KML maps with the current version, but not with the coloured depth areas - just a single fill colour, with (optionally) colour-coded contours (see
this post for some examples).