Take apart the panel that you have installed. Get a connector to fit the PVC coming in the top of the panel. Get a panel that is at least as tall as the siemens you are taking out, you will need this because you have already stripped wires coming from the top and bottom. Main lug or main breaker really dosen’t matter, the overcurrent protection is being provided by the 125A in your main panel, get whatever is cheaper. Make sure the panel you get has long busbars on each side (a full size GE powermark gold mod 6 would be good I think) so your already cut wires will reach. Get some plastic romex connectors, they are easier to work with. Make sure you remove the jumper from the neutral bar to the ground bar, this will make your panel a subpanel. Get some #10 sheetmetal screws, as drywall screws or deck screws aren’t acceptable for mounting a panel where I come from, maybe its ok for you. I would get some 1/0 SER Aluminum to feed that panel but you could use PVC with THHN, I think SER is easier and cheaper. Make sure you secure those homeruns within 12 inches of the panel. If you keep them to the side you could use plastic cable stackers (depending on how many homeruns you have) or you might want to turn a 2x4 sideways and mount it between the two studs to staple to. I would stay away from siemens, replacement breakers are always easier to find for GE or Cutler Hammer or Square D.
Those are some things that you would only know from doing it, but I have to ask as an EE, don’t you spec out what panels to use on jobs. Most sets of prints I see have panel schedules with type of panel (MLO or Main Breaker) listed.
