I have installed 3 uAvionix Echo UAT systems in RV's. In all three I used the external combination skyFYX-EXT GPS/Antenna unit mounted under the top cowling. Two at the firewall on a shelf and one on an engine mount. If you go with the under-the-cowl option for the GPS unit keep the it away from any high seat source. In all instances Echo was paired with a digital Garmin GTX 327 Mode C transponder in wireless sense mode, not direct wired with the Mux cable option. I have never had any issue with the wireless sensing. Here are some photos. First two are of my RV-3B and third one is an RV-7A:
One of the things I really like about the Echo UAT over the wing tip SkyBeacon or TailBeacon models is its small size and the fact it can be installed anywhere without disrupting wingtip or tail lights. More importantly it can be powered without reliance upon the Nav light circuit. I have installed two SkyBeacons and one TailBeacon and think the advertised time listed for the work is way too low to match real-world issues.
I also still find it baffling that uAvionix designed the SkyBeacon and TailBeacon without a separate wires for ADS-B power and Nav power. If they had done the separate ADS-B power wire it could still be installed the same way as the current one-wire Nav/ADS-B unit by connecting both power supply wires together at the unit. But then the installer could also opt to have a separate power leads for each so ADS-B out could be installed without having to have the Nav lights on all the time. Although not relevant for RV's, this single-lead ADS-B/Nav circuit really comes home to roost when a Beacon type device is installed in a retractable gear aircraft that dims the gear lights automatically when the Nav lights are turned on. In Pipers the pilot cannot see the gear lights during the day when they are dimmed. So there is a catch 22 as the pilot decides whether it is more important to see the gear lights or more important to not turn off the nav lights and violate the "always on" ADS-B mandate. A reasonable pilot would turn off the nav lights momentarily to see the gear lights and the ADS-B system nanny computer be damned.
Yes, there are also much more advanced and feature-rich 1090ES-based ADS-B out units, most of them fancy expensive transponders. They have their place but you seem to be asking for the easiest and cheapest option.
In summary, the simple and cheap ADS-B UAT out unit I recommend for experimental aircraft is the uAvionix Echo UAT/sky-FYX-EXT combination. For type certified I recommend the Garmin GDL 82 UAT out unit. And in both cases matched with an existing or used digital transponder like the Garmin GTX 327 but definitely in the case of the GDL 82. You can use an older tube-type transponder initially but you will likely be replacing it in a year or two after the ADS-B unit wears it out demanding information from it multiple-times a second. A digital transponder can handle this indefinitely. Used GTX 327 transponders are usually available on the used market for around $400 including tray and connector so a Echo+skyFYX-EXT bundle combined with a used GTX 327 if needed will come in at approximately $1800 component costs.