Looks like it would take standard low profile cards according to QNAP's recommended list below.
Compatibility List - QNAP
Recommended
Expansion Card for TVS-682
Brand
Brand Model Specification Note
QNAP LAN-10G2T-U Dual-port 10 GbE network expansion card for rack mount model, low-profile bracket, cat6A cable
Emulex OCe11102-IM Dual port 10 Gigabit network adapter , short range optical cable
Emulex OCe11102-NX Dual port 10 Gigabit network adapter , SFP+ direct attach copper cable
Emulex Oce11102-NT Dual port 10 Gigabit network adapter, 10GBASE-T, cat6 cable
QNAP LAN-10G2SF-MLX Dual-port 10 Gigabit network adapter, SFP+ interface
QNAP LAN-40G2SF-MLX Dual port QSFP 40 Gigabit network adapter
Mellanox MCX311A-XCCT Dual port 10 Gigabit network adapter , SFP+ direct attach copper cable
Mellanox MCX313A-BCCT Network Interface Card 40/56GbE Single-Port QSFP
Mellanox MCX314A-BCCT Network Interface Card 40/56GbE Dual-Port QSFP
QNAP LAN-10G2T-X550 Dual-port 10 GbE network expansion card , cat6A cable
QNAP LAN-1G2T-I210 Dual-port 1 GbE network expansion card