I would get the standard Monitor Panel Well nut (3/8 inch hole to be drilled), insert the well nut and then liberally Loctite the screw threads and install the threaded screw and let sit overnight. There is also a Well nut that does NOT have the removable screw, but threads that will seal this type tank. Doug
This is the 2nd one I would use.
http://www.amazon.com/gp/product/B007HRRC6E/ref=pd_lpo_sbs_dp_ss_1/189-9626172-2967537?pf_rd_m=ATVPDKIKX0DER&pf_rd_s=lpo-top-stripe-1&pf_rd_r=0RHQY254HDJ5TMGCAJTQ&pf_rd_t=201&pf_rd_p=1535523722&pf_rd_i=B007NZVK04
This is the first one
http://www.amazon.com/Tank-Well-Assembly-Level-Sensors/dp/B007NZVK04
