Make a simple box spar and alignment pins to go thru the fuselage and pin the wings to it. Can be fancy and use carbon fiber of be like us and improvise with foamboard. Having the dimensions of the aircraft would help get better answers to adjust for strength and weight more accurately then just a general idea like I gave.
Yeah a simple carbon tube spar about 1/2 to 2/3 the total wingspan and another "pin" spar that goes thru the fuselage and sticks out a few inches either side to hold the wing in place so it doesn't pitch up or down under load should be fine. then you could drill thru the wing and the main spar and pin or bolt the wings in place so they don't slide away from the fuselage.