Piggy back fuse holders are not a good long term solution. The fuse box contacts are not designed to resist more than the weight of the fuse itself, not a dongle hanging off (with the vibrations of driving, especially) nor are they designed for constant insertion/removal cycles to swap the dongle and original fuse back and forth.
How convenient does this trickle charge hookup really need to be? I mean if driving infrequently enough to need the trickle charger, it doesn't seem a big deal to pop the hood, but if you really want some interior access method (then how is the wire going to get outside the vehicle, leave a gap in the door seal for the wire to exit?), then I would sooner tap into the wire itself, coming off the fused end of the circuit, that + and a chassis ground going to a DC barrel connector.
Next I would wonder if the interior is really the right place to put that, or if better to get a weatherproof holder for it and mount somewhere like in the grill... considering you're in Tx, probably not getting a winter iced-up grill very often to impede access to it. If doing something like that with grill access, no need to tap into the fuse box, just make the shorter run straight to the battery with an inline fuse near the battery.
Another option, rewire the lighter outlet to always be live, or depending on the circuit (what else it feeds), a parallel run to the outlet with a switch to make it live, and a blocking diode so it doesn't back-flow into anything else on the same circuit IF there is anything else on same circuit, OR find a place to put in another lighter outlet, make a (new, fuse added) run from a live circuit to that, though if there is any chance that at some later date, someone might accidentally plug a higher current device into that outlet, then better to make that a straight run to the battery with a series fuse, instead of the potential of pulling more current on the circuit tapped into, than it was designed for.
I'd do just about (within reason) anything instead of a piggy back fuse holder. In the grand scheme of vehicle maintenance and repairs, it is not that much work (or expense) to figure out and hook up something else, as long as you can DIY instead of paying a 3rd party for a custom mod.
