Terminal 1:
user@host$ unshare --user --map-root-user --net --mount
unshared@host$ echo $$ > /tmp/pid 

Terminal 2:
user@host$ slirp4netns --configure --mtu=65520 --disable-host-loopback $(cat /tmp/pid) tap0
Starting slirp
* MTU:             65520
...

Terminal 1:
unshared@host$ ip a
1: lo: <LOOPBACK> mtu 65536 qdisc noop state DOWN group default qlen 1000
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
3: tap0: <BROADCAST,UP,LOWER_UP> mtu 65520 qdisc fq_codel state UNKNOWN group default qlen 1000
    link/ether c2:28:0c:0e:29:06 brd ff:ff:ff:ff:ff:ff
    inet 10.0.2.100/24 brd 10.0.2.255 scope global tap0
       valid_lft forever preferred_lft forever
    inet6 fe80::c028:cff:fe0e:2906/64 scope link 
       valid_lft forever preferred_lft forever
unshared@host$ echo "nameserver 10.0.2.3" > /tmp/resolv.conf
unshared@host$ mount --bind /tmp/resolv.conf /etc/resolv.conf
unshared@host$ curl https://example.com